Boron nitride has many forms. There are at least four crystalline forms of boron nitride: hexagonal, rhombohedral, cubic, and wurtzite. Hexagonal and rhombohedral boron nitride (hBN & rBN) are structurally similar to hexagonal and rhombohedral graphite, respectively, with sheets of sp.sup.2 bonded hexagonal arrays of boron and nitrogen atoms stacked as ABA or ABCA along the c-axis. HBN and rBN are extremely soft, electrical insulators with poor thermal conductivities. They are mainly used as high temperature solid lubricants, refractory materials, and as starting materials from which cubic or wurtzite BN are formed at high temperatures and pressures. On the other hand, cubic and wurtzite boron nitride (cBN & wBN) are structurally similar to diamond and lonsdalite, with tetrahedrally coordinated frameworks of sp.sup.3 bonds. Consequently, cBN is an extremely hard (second to that of diamond), electrical insulator with an excellent thermal conductivity (also second to that of diamond). CBN is currently used as a powder for abrasive processes, and as sintered ceramics for sawing, cutting, and crushing applications. Sintered cBN is particularly useful as inserts in high speed machining of hardened steels, chilled cast iron, carbides, and nickel or cobalt-based superalloys. Unlike diamond, cBN has a low reactivity with iron and steels, and its high thermostability in oxidizing conditions makes it a better candidate than diamond for engineering materials. CBN has potential applications that include wear resistant and protective coatings, thermal head sinks for electronic devices, electrical insulators in silicon based devices, lattice matched substrates for diamond growth, and when electrically doped as high temperature and high power electronic and optoelectronic devices. However, cBN films are brittle at grain boundaries and do not adhere well to many surfaces. These cBN films tend to pop off in wear resistance applications.
Not much is known about the properties of the wurtzitic form of boron nitride.
Another very common form of BN is a quasi-amorphous phase called turbostratic boron nitride (tBN). The sp.sup.2 bonded tBN actually has short-range hexagonal and/or rhombohedral order, but there is no well-established long range stacking sequence. A tBN phase is usually observed in BN films grown by physical vapor deposition (PVD) processes.
HBN is most commonly grown either in film or powder form by chemical vapor deposition (CVD). Typically, reactants such as BCl.sub.3 and NH.sub.3 are combined on substrates heated above 1000.degree. C. in appropriate concentrations to form hexagonal boron nitride. The substrate temperature can be reduced considerably when the CVD process is excited by a plasma process such as hot filament, microwaves, or radio frequency waves.
CBN films and powders are more difficult to produce than hBN. For example, cBN films have only been grown successfully when a physical vapor deposition (PVD) process such as sputtering, e-beam evaporation, or laser deposition is modified with a beam of nitrogen or nitrogenic ions irradiating the target. Although the mechanism for the growth of cBN by PVD is still not well understood, it is generally accepted that only ion-assisted PVD (IA-PVD) processes will yield films with any sp.sup.3 bonding. BN films grown by these IA-PVD techniques have a layer of amorphous BN at the substrate interface. About 5 nm from the interface, a tBN phase starts to emerge. The tBN phase is characterized by the hexagonal BN sheets oriented normal to the substrate. Beyond the tBN phase, a cBN phase can be present. Usually, the cBN phase is poorly crystallized and randomly oriented, although if the ions have a sufficient energy, the cBN phase with have larger crystallites and become preferentially oriented. However, these processes are not easily scaled to high volume manufacturing (as in the case of laser deposition), or do not make films with adequate adhesion (sputtering and e-beam evaporation).
Powders and crystals of cBN are produced by heating hBN and rBN to very high temperatures (2000.degree. K) under considerable pressures (11 Gpa). The formation of cBN in this direct conversion occurs via a destructive-reconstructive diffusion-like process. The high activation barrier is lowered by the use of an hBN or rBN starting material with a poor crystallinity and small particle size. The destruction of the lattice and the diffusion of atoms is easier in a material containing a high concentration of defects. The temperature and pressure of the transformation can be reduced slightly by adding a catalyst such as an alkaline or alkaline earth metal. In this catalyzed process, compounds are added to hBN in order to reduce the high activation energy through a eutectic interaction with the BN. The driving force for the formation of cBN is the solubility difference between hBN and cBN varieties in the eutectic flux. The catalyzed process is the main method used to produce cBN at the industrial scale.
The present invention overcomes some of the disadvantages of cBN applications, and the PVD method of producing cBN films.